Vapor phase coating of molybdenum articles



Ilune 28, 1955 E. WAINER ETAL VAPOR PHASE COATING OF MOLYBDENUM ARTICLES Filed June l0, 1949 f HL 55 nitel States VAPR PHASE CG'HNG 0F MOLYBDENUM ARTICLES 7 claims. (ci. 117-107) The present invention relates to a method for coating articles employing a vapor phase deposition of the coating material onto the surface of the article.

More particularly, the present invention relates to a method of coating a molybdenum article in a vapor phase coating process to enhance the resistance of the body to the effects of high temperatures and corrosive atmospheres. The process of the present invention is particularly applicable in the manufacture of turbine buckets for use in jet turbines and the like.

iet engines or the like are usually provided with an axial ow turbine operated by exhaust gases which drive a blower furnishing air to the burners. Such turbines operate at extremely high temperatures, and one of the major diculties encountered in the manufacture of jet turbines has been the provision of suitable material for bucket blades which can withstand the effect of such high temperatures. The turbine bucket will normally be exposed to temperatures in the range from about 1600 F. to 2G00 F., and the bucket must have suiiicient strength, toughness, creep resistance, and resistance to the oxidizing gases present in the turbine engine to enable the bucket to operate efficiently without deformation or corrosion.

In addition to turbine buckets, articles produced by the present invention may be employed under conditions of higher temperature and lower stress than exist in a gas turbine bucket. One such application occurs in nozzle diaphragm vanes in gas turbines which must withstand very severe conditions of temperature and thermal shock but at a relatively low stress.

One metal which exhibits excellent properties of strength, toughness, and creep resistance at an elevated temperature is molybdenum. However, metallic molybdenum itself cannot be used since the trioxide of molybdenum, which is formed under the oxidizing conditions present in the turbine, sublimes at a temperature of about l463 F. at an extremely rapid rate. This phenomenon gives rise to a characteristic smoking when bodies of molybdenum are heated to temperatures above about 1463" F., resulting in the complete disappearance of the molybdenum in a matter of minutes.

To overcome this diculty, we have herein provided a process for coating molybdenum base articles to provide thereon a tough, corrosion-resistant coating impervious to the effect of oxygen and other gases for the purpose of protecting the molybdenum body.

In essence, the invention comprises passing a volatilized compound of a metal selected from the group consisting of silicon, zirconium, and aluminum in a stream of hydrogen in contact with a molybdenum article at elevated temperatures for a period of time suiiicient to cause the coating metal to become bonded to or otherwise react with the molybdenum. The particularvcoating metals mentioned above have been found to be especially well adapted for coating molybdenum, since the oxides of these metals which are formed under the oxidizing conditions of turbine operation and the interrnetallic comarent`A vpounds produced have the ability to protectl the molybdenum surface from the corrosive effects encountered under operating conditions of a turbine engine.

While commercial molybdenum may be used satisfactorily for the purposes or' this invention, it is sometimes desirable to add minor amounts of elements such as tungsten in amounts up to about 2% to raise the reerystallization temperature of the body.

The structures produced by the coating processes of the present invention exhibit definite evidence of the presence of intermetallic compounds formed by thereaction of the coating metal with the molybdenum base. Where the molybdenum article is coated with silicon, intermediate layers of compounds such as MoSi2 and MozSi are formed between the outer silicon coatingY and the molybdenum base. These compounds apparently `have the ability to prevent diffusion of corrosive gases into the molybdenum body.

The coatings produced in accordance with this invention are integrally bonded to the base metal and cannot be stripped mechanically from the body metal, as is the case of coatings applied by electroplating ordipping. Further, the coatings are inert with respect to the molybdenum base metal and show no evidences of reaction with the base metal after the original deposition.

The vapor phase deposition processes described herein have complete throwing power, i. e., a uniform coating can be deposited over the entire surface of the article regardless of corners, grooves, or other irregularities in the surface of the article. This is not true of other types of coating procedures.

The initial reaction in the coating process involves the decomposition of a compound of the coating metal, preferably a halide, by means of various reactions. Some of the compound is undoubtedly decomposed by the high temperatures present in the coating zone. A large part of the halide, or other reducible compound, is also reduced by the stream of hydrogen in which the compound is introduced into the coating zone, and by the hydrogen atmosphere within the zone itself.

Another reaction which occurs is the metathetical reaction between the coating metal compound and the molybdenum wherein the coating metal is deposited on the molybdenum with the formation of a volatile molybdenum compound in the exchange reaction. By this mechanism, a coating may be applied without increasing the dimensions of the article.

The three reactions mentioned above may be described by the following equations, where aluminum chloride is used in the starting material:

The decomposible compound to be used in accordance with the present invention is preferably a halide, for example, silicon tetrachloride, trichlor silane, silicon tetrabromide, tribromo silane, silicon tetraiodide, aluminum chloride. aluminum bromide, aluminum iodide, zirconium chloride, zirconium bromide or zirconium iodide. `Or` the compounds listed, the silicon tetrachloride is preferred, since the compound exists normally in liquid state, thus facilitating its introduction into the coating zone by the hydrogen gas. Where solid compounds such as aluminum chloride and zirconium chloride are used, it is necessary to at least partially Vaporize the compound before introducing the same into the hydrogen stream.

Prior to coating, the molybdenum article is cleaned by suitable means, such as grit blasting, pickling, neutralizing, washing and drying, all these methods being conventional and well known to those skilled in the art.

After such cleaning, the articles to be coated are placed passed into the furnace over the articles to be treated in a stream of dry pure hydrogen gas. Passage of the gas containing the vaporized coating compound is continued for a relatively long period of time, normally on the order of two to eight hours. During such treatment, the volatilized coating compound decomposes by one of a number of reactions, previously described, and is deposited with and firmly bonded to or into the surface of the molybdenum article. It has been found that this procedure when used to apply a coating of silicon to a molybdenum article results in a coating which penetrates into the body, forming a definite silicon gradient from the exterior of the article to points just beneath the surface of the article. The normal depth of the silicon layer will be on the order of 0.0003 to about 0.003 inch. The outermost layer is normally substantially pure silicon, the next immediate layer is probably a molybdenum-silicon compound having a relatively high silicon content, possibly n An object of the present invention is to provide a n method for rendering molybdenum articles resistant to the effects of corrosive atmospheres at high temperatures.

Another object of the present invention is to provide a method for depositing corrosion-resistant surfaces on` molybdenum turbine buckets.

A further object of the present invention is to provide a method for coating molybdenum articles by means of a vapor phase process to thereby produce a corrosion?.

resistant, strong, tough, molybdenum article.

A further description of the present invention will be made in connection with the attached sheet of drawings, in which:

Figure 1 is a flow sheet showing, in general the variousstages in the coating process;

Figure 2 illustrates the modification wherein a normally solid compound of the coating metal is to be used;

Figure 3 is a plan view of a turbine bucket coated in accordance with the present invention;

Figure 4 is a top plan view of the turbine bucket shown in Figure 3; and

Figure 5 is a drawing of a photomicrograph, taken at a magnification of 500x, showing the crystal structure of the molybdenum articles coated in accordance with the;-V

present invention.

As shown on the drawings:

Numeral 10 denotes a source of purging gas, which may be nitrogen, or an inert gas such as argon, neon, krypton, helium, and the like, which is passed into a purification zone 11 where moisture and other contaminants are removed. The purification zone 11 may consist essentially of a supply of liquid sulfuric acid through which the purging gas is bubbled. The thus purified gas is next introduced into a heated furnace 12 which sur# rounds a furnace tube 13, control of the gas ow being regulated by means of a valve 14. Disposed within the furnace tube 13 are a plurality of boats 15 which carry a number of turbine buckets 16 or other articles to be coated. The temperature of the furnace is preferably in the range from l600 F. to l800 F., although temperatures up to 2300 F. may be used, provided the 4 coating time is decreased at the more elevated temperatures.

A source of hydrogen gas 17 is introduced into the furnace zone through a purification zone 18 which may consist of various desiccating stages, such as, for example, packed columns of silica gel, calcium chloride, or liquid sulfuric acid. Flow of the hydrogen gas into the furnace tube 13 is controlled by means of a valve 19. In addition, a second source of hydrogen 20 and an accompanying purification zone 21 are also provided to carry the vaporizable compound of the coating metal into the furnace zone.

Reference numeral 22 denotes a source of the coacting metal compound, which, in the case of silicon' tetrachloride, may be a liquid pool through which the hydrogen gas is bubbled, causing the carrier hydrogen gas to pick up the vapors of the coating compound. The resulting gaseous mixture is next passed through valve 23 and into the furnace tube 13 together with the hydrogen appearing from the source i7.

The furnace tube is also provided with an exhaust 24 for carrying away the hydrogen gas, which may be recycled or burned.

The buckets 16 are kept within the furnace tube 13 for a time suicient to cause a coating of the desired thickness to become deposited thereon.

In case a solid coating compound is to be used, for example zirconium chloride or aluminum chloride, the modification of the apparatus shown in Figure 2 may be used. As shown in Figure 2, the furnace tube 13 is provided with a refractory container Z5 which holds the powdered coating compound 26. A source of heat, such as induction coils 27, surrounds the container 25 to vaporize the coating compound 26. It is preferable to supply enough heat to the coating compound 26 to produce a vapor pressure of about one-half atmosphere due to the vaporized coating compound. The vaporized coating compound is picked up by means of the hydrogen gas and passed into the furnace zone as in the case of the normally liquid coating compounds.

Figures 3 and 4 illustrate a finished turbine bucket coated in accordance with the process of the present invention, consisting of a blade portion 28 and a firtree root 29 for securing the same in the turbine wheel. The bucket is coated with an impervious coating 3l) of a metal of the type described to a depth between .0003 and..003 inch.

Figure 5 illustrates the crystal structure of the coated article produced in accordance with the present invention. The outer layer of the article consists essentially of a layer 31 of pure coating metal, for example, silicon. immediately underlying this silicon layer is a phase which apparently is a mixture of silicon and compounds of molybdenum and silicon relatively rich in silicon, such as, for example, MoSiz. This phase is represented by the reference numeral 32. Underlying the phase 32 is a second phase 33 of relatively massive crystals which probably comprises a compound of the type MozSi. The body metal 34 is essentially molybdenum crystals.

From the foregoing it will be apparent that we have herein provided a novel method for coating of molybdenum articles to produce a gas-impervious, temperature-resistant, and corrosion-resistant outer surface which is firmly bonded and alloyed to the molybdenum article -with the result that the molybdenum article may be used at temperatures far exceeding those to which molybdenum itself can be effectively utilized.

It will be understood that modifications and variations may be effected without departing from the scope of the novel concepts of the present invention.

We claim as our invention:

l. The method of coating a molybdenum article which comprises passing a volatilized compound of a metal selected from the group consisting of silicon, zirconium, and aluminum in a stream of hydrogen in contact with said molybdenum article at a temperature between 1600 ii. and 23O F., and maintaining said article in contact with said stream for a period of time of at least two hours and sulicient to cause said metal to form oxidation-resistant intermetallic compounds between the metal arti the molybdenum surface.

2. The method of coating a molybdenum article which comprises passing a volatilized halide of a metal selected from the group consisting of silicon, zirconium and aluminum in a stream of hydrogen in contact with sni'i molybdenum article at a temperature between 1600 F. and 2300 F., and maintaining said article in Contact with said stream for a period of time of at least two hours and sucient to cause said metal to form oxidation-resistant intermetallic compounds between the metal anl the molybdenum surface.

3. The method of coating a molybdenum article which comprises passing a volatilized halide of a metal selected from the group consisting of silicon, zirconium,

and aluminum in a stream of hydrogen in contact wib l said molybdenum article at a temperature between 1600" F. and 2300 F., and maintaining said article in contact with said stream for a period of time of at least two hours and sufficient to cause the formation of a coating including oxidation-resistant intermetallic compounds of tfi metal and the molybdenum having a thickness between .0G03 and .003 inch.

4. A method of producing a corrosionand temperature-resistant molybdenum article which comprises forming molybdenum into a predetermined shape, passing a. volatilized halide of a metal selected from a group consisting of silicon, zirconium and aluminum in a stream of hydrogen in contact with said shape at a temperature between 160D" F. and 2300 F., and maintaining said article in contact with said stream for a period of tinte ,z

of at least two hours and suicient to cause the formation of a coating including oxidation-resistant intermetallic compounds of the metal and the molybdenum surface, said coating having a thickness between .0003 and .O03 inch.

5. The method of producing a corrosionand temperature-resistant molybdenum article which comprises forming molybdenum into a predetermined shape, passitil I1 forming molybdenum into a predetermined shape, passing volatilized silicon tetrachloride in a stream of hydrogen in contact with said shape at a temperature of 1600 F. to 1800 F., and maintaining the said article in contact with said stream for a period of time of at I least two hours and suicient to cause the formation of a coating including oxidation-resistant intermetallic contpounds of silicon and molybdenum at the molybdenum surface, said coating having a thickness between .0G03 and `0G31 inch.

7. The method of producing a corrosionand temperature-resistant molybdenum article which comprises passing volatilized silicon tetrachloride a stream of hydrogen in Contact with a molybdenum article at a temperature ot l6G0 to 1800 F. to thereby decompose said silicon tetrachloride and maintaining said article in Contact with said stream for a period of at least two hours and until the surface of the molybdenum rcacts with the silicon resulting from the decomposition of the said silicon tetrachloride to form oxidation-resistant intermetallic compounds of molybdenum and siiicon at the molybdenum surface thereby producing a silicon coated molybdenum article characterized by the presence of intermetallic compounds of silicon and molybdenum having a proportionately smaller silicon content from the exterior silicon layer to the molybdenum body.

References Cited in the le of this patent UNlTED STATES PATENTS 1,899,569 Howe Feb. 28, 1933 2,313,410 Walther Mar. 9, 1943 2,491,284 Sears Dec. 13, 1949 

7. THE METHOD OF PRODUCING A CORROSION- AND TEMPERATURE-RESISTANT MOLYNBDENUM ARTICLE WHICH COMPRISES PASSING VOLATILIZED SILICN TETRACHLORIDE IN A STREAM F HYDRGEN IN CNTACT WITH A MOLYBDENUM ARTICLE AT A TEMPERATURE OF 1600* TO 1800* F. TO THEREBY DECOMPOSE SAID SILICN TETRACHLRIDE AND MAINTAINNG SAID ARTICLE IN CONTACT WITH SAID STREAM FOR A PERIOD OF AT LEAST TWO HOURS AND UNTIL THE SURFACE OF THE MOLYBDENUM REACTS WITH THE SILICON RESULTING FROM THE DECOMPOSITION OF THE SAID SILICON TETRACHLOORIDE TO FORM OXIDATION-RESISTANT INTERMETALLIC COMPOUNDS OF MOLYBDENUM AND SILICN AT THE MOLYBDENUM SURFACE THEREBY PRODUCING A SILICON COATED MOLYBDENUM ARTICLE CHARACTERIZED BY THE PRESENCE OF INTERMETALLIC COMPOUNDS OF SILICN AND MOLYBDENUM HAVING A PROPORTIONALLY SMALLER SILICN CONTENT FROM THE EXTERIOR SILICON LAYER TO THE MOLYBDENUM BODY. 